In the state of the art, the housings used in rolling stands to support and position the rolls have a standardised conformation, substantially quadrilateral, wherein it is possible to distinguish two vertical uprights and two horizontal cross-pieces.
At present, these housings are produced using mainly two methods.
A first method provides to cast steel or iron to obtain the housings in their definitive configuration, in single pieces; a second method provides to obtain the housings by means of full penetration welding of steel casts forming the several component parts.
Both these methods, however, have some disadvantages.
To be more exact, the first method of producing housings entails a problem of quality connected with the difficulties of obtaining large casts without impurities, porosity and blowholes which are typical of the casting process itself.
These quality defects, given the geometry of the housings, tend to collect in the proximity of the connection zones between the uprights and the cross-pieces where usually the mechanical stresses to which the housings are subjected during working conditions are greater compared with the other areas of the housing.
Moreover, because of the different cooling speed of the upright area and the cross-piece area, residual tensions are created in the aforesaid areas which are added to state of tension due to the working load.
On the one hand this reduces the safety factors and, on the other hand, it reduces the length of time the housings can function at maximum efficiency.
With this first method, moreover, the housing is made using a single material, for example steel or cast iron, while the stresses in the various sections of the housing are somewhat different, reaching higher values at the pass-line, that is, the ideal line of passage of the rolled stock, and in correspondence with the connection zones between the uprights and the cross-pieces.
Therefore, this would require zones with a diversified mechanical resistance according to the stresses which they have to bear.
Moreover, any inner defects, such as the inclusion of earth, worm holes and porosity can cause cracks, while the housings are in use, which would negatively affect the housing's ability to resist fatigue.
It should also be noted that, most times, these inner defects are very difficult to detect, especially because of the great thickness of the housing, the grain size of the material used and the surface finish which usually leaves the surfaces of the housings in their rough cast condition.
The second method, that is, the one which provides to make housings consisting of component parts made autonomously and then welded together with full penetration steel casts, eliminates in part the disadvantages of the first method, particularly those connected to the high dimensions of the casting.
Full penetration welding means welding which allows to completely restore the resistant section and wherein, since there is a complete continuity of the two connected pieces, there is a distribution of loads similar to that of a whole piece without any weld.
The second method, however, also has some disadvantages which negatively influence the resistance of the housings, and reduce their working life.
To be more exact, with this method three distinct zones are created: one welded, one cast and one thermally altered, each of which is characterised by different mechanical characteristics.
The high quantity of heat required by the full penetration welding process entails a considerable growth in the grain of the material used, with a consequent decline in the mechanical characteristics and particularly of the resilience and the resistance to fatigue.
Moreover, in order to guarantee a full penetration of the housings, a high volume of weld is used, which causes high shrinkage tensions which can be eliminated only by means of a normalisation treatment in the furnace. However, this treatment entails a considerable cost, caused by the lengthening of the production process inasmuch as, if it is to be efficient, the housing may have to remain in the furnace for more than a hundred hours.
With full penetration welding, moreover, there may arise problems connected with the elimination of the slag or the fact that one of the two ends to be welded may not be locally cast.
Inclusions of slag and stuck welds which derive therefrom cannot be eliminated with the heat normalisation treatment, and can cause the formation of cracks and breakages which are extremely difficult to detect because of the great thicknesses and the surface finishing of the housing, which is normally left rough.
Such cracks can in the long term even lead to a decline in the mechanical characteristics and therefore to a reduced safety in the functioning of the housing.
WO-A-96/26022 describes a housing for rolling stands consisting of uprights and cross-pieces which in turn consist of two parts, respectively an outer part and an inner part, mutually connected by means of reinforcing ribs; in the case of the vertical uprights the reinforcing ribs may also be absent.
The independent parts which constitute the uprights and cross-pieces define spaces between them, which allow to install conductors, electric cables and devices to support and move the rolls.
The two parts which make up the uprights are welded independently to the two parts which make up the cross-pieces, but with this solution a discontinuous zone is created in the center due to the lack of structural elements suitable to make the stand monolithic once it has been assembled.
The stand described in WO'022 is configured as a pair of rings, respectively inner and outer, constrained together only in correspondence with the vertical median axis and, optionally, astride the horizontal median axis.
This solution does not guarantee an adequate resistance to stresses, particularly in the case of unbalanced axial forces or in the case of pulsating stresses, and it is particularly susceptible to torsion on the vertical plane which affect the welds and, in the long term, compromise their efficiency.
With the welding between uprights and cross-pieces as used in WO'022, the inner zones near the root of the welds, which may have cracks or defects, are not removed with subsequent processing and therefore, with time, may lead to structural weakening and damage.
The reduced thickness of the uprights, given the same outer dimension, causes a reduction in the resistant section which is often unacceptable from the point of view of planning, since this reduction affects the average stress which can be supported by the uprights. Moreover, the welding procedure causes difficulties in the correct centering of the elements to be welded.
A further disadvantage is that the faces of the housings do not have a continuous surface, which creates problems when functional assemblies and accessories have to be attached to said faces.
The present applicant has devised. and embodied this invention to overcome the shortcomincgs of the methods of production as are known in the art, and to obtain further advantages as will be shown hereafter.